1. Field of the Invention
The present invention relates to an oximeter and, more particularly, to an apparatus for measuring the degree of oxygen saturation of arterial blood (hereinafter also referred to as SaO.sub.2) a non-invasive manner.
2. Description of the Prior Art
The oximeter of non-invasive type generally has a source of a light from which a light is directed to a body member of the subject, such as a finger or ear lobe. The light passes through the body member and is then detected by the detector. The amount of light absorbed by the body member as light is transmitted through it is a function of the attenuation which is dependent on the amount of oxygenated hemoglobin in the arterial blood in the body member. Accordingly, by measuring the transmitted light, oxygen saturation (SaO.sub.2) in the arterial blood can be determined.
According to the prior art oximeter of the above described type, the active motion of the body member, such as a movement of the body member or an irregular pulse of blood pressure causes an undesirable change in the light amount measured by the detector, thus resulting in undesirable noise signal in the detected signal. Since it is impossible to stop the active motion of the body member, a means for detecting the active motion is provided. When the detecting means detects the active motion, the measured result is ignored. Here, the problem is to provide such a detecting means which has a high accuracy in detecting the active motion in the body member.
According to a first prior art oximeter, the active motion is detected by the detection of a sudden and great change in the amplitude of the pulse signal. In this case, the active motion with a sudden change can be detected, but the active motion that changes gradually and incessantly can not be detected.
According to a second prior art oximeter, the active motion is detected by the detection of a difference in the step-up time, or step-down time, between the pulse signal under active motion and pulse signal under a steady condition. However, because of wide variations between patients in such a step-up time or step-down time, and even in the same patient due to the environmental change, the accuracy of the detected result is very poor.
According to a third prior art oximeter, the active motion is detected by the detection of change in the pulse rate. This oximeter, however, can not detect the active motion when the active motion occurs periodically.
An improved oximeter, which is invented by one of the present invention, Kenji HAMAGURI, and is assigned to the same assignee, is disclosed in Japanese Patent Laid-open Publication (Tokkaisho) No. 55-120858. According to this publication, the light passed through the body member is detected at three different wavelengths. Using the detected amount of light Ea1, Ea2 and Ea3 at three different wavelengths, the amount of absorption Eb1, Eb2 and Eb3 by the blood in the body member is calculated for each of the three wavelengths. Then, differences Ec1 and Ec2 are obtained through the calculations: EQU Ec1=Eb1-Eb3
and EQU Ec2=Eb2-Eb3.
Then, using the obtained differences Ec1 and Ec2, the degree of oxygen saturation of arterial blood (SaO.sub.2) is obtained.
Even in this improved oximeter, since the oxygen saturation (SaO.sub.2) is calculated using the signal which has been already influenced by the active motion of the body, the influence of the active motion can not be completely removed from the result obtained from the improved oximeter. In the case where the action motion is great, the obtained result will be very poor in reliability.
Also, according to the prior art of noninvasive type, a digital display device and/or an analog current meter are employed for the indication of oxygen saturation (SaO.sub.2) and pulse rate in the instantaneous values or in the averaged values. By the continuous watch on the patient's oxygen saturation using the oximeter of noninvasive type, the sudden condition change of the patient can be catched easily. However, with the digital display device or analog current meter itself, it is difficult to know the gradual condition change of the patient.
Furthermore, according to the prior art oximeter, the indication through the digital display device and/or the analog current meter is effected after a certain period of time, but when it is detected that the measured result does not have a sufficient accuracy, the indication is skipped. When the skipping takes place for a number of times frequently, the indication on the display is effected irregularly.
Accordingly, because of the disadvantages mentioned above, it has been difficult to obtain accurate and trustworthy results from the oximeter of the prior art.